Process for Manufacturing Cold-Formed Precision Steel Pipes

ABSTRACT

The invention relates to a process for manufacturing cold-formed, in particular cold-drawn, precision steel pipes for application in particular as pressure-operated cylinder pipes with optimum addition of one or several alloying elements as well as impurities caused by melting. A seamless, hot-formed pipe blank or welded pipe blank made from a hot strip with defined starting condition is hereby drawn in one pass or in several passes into a finished pipe, and the pipe undergoes a heat treatment before the finishing pass.

The invention relates to a process for manufacturing cold-formed, in particular cold-drawn, precision steel pipes according to the preamble of claim 1.

Involved in particular in this context are precision steel pipes according to DIN EN 10305, part 1 and 2, which are under high internal pressure during operation and find application for example as cylindrical pipes in the hydraulic or pneumatic fields.

The basic manufacturing process of seamless or welded, cold-drawn precision steel pipes is described for example in the Stahlrohr Handbuch [Steel Pipe Handbook], 12. ed. 1995, Vulkan Verlag Essen.

Pipes manufactured in this way are characterized in particular by narrow wall thicknesses and diameter tolerances.

Starting product may either be a seamlessly produced hot-rolled pipe blank or a pipe blank made from a hot strip by means of high frequency induction welding (HFI welding).

This pipe blank, labeled also as hollow, is drawn in the following cold drawing process, which includes one or more passes, to the required final size (diameter, wall thickness) for the finished pipe.

Cold forming causes the material to solidify, i.e. yield point and strength thereof increase while elongation and toughness thereof become smaller at the same time.

This is a desired effect for many applications. As a consequence of the reduced deformation capability, it is, however, necessary to execute in some instances a recrystallizing heat treatment before carrying out further forming processes, so that the material can be cold formed again for the next drawing process.

The properties of precision steel pipes made in this way are described in DIN EN 10305 part 1 and 2.

Unalloyed quality steels up to E 355 as well as higher strength grades up to StE 690 are used as steel grades.

In order to use such pipes under high pressure, e.g. as hydraulic cylinder pipes, they have to meet high standards as far as their toughness is concerned. Hydraulic cylinders control movement patterns of many devices and machineries which are used, i.a., also outdoors at great temperature fluctuations.

When exposed to temperature conditions of up to −20° C., risk of harm to persons and material cannot be absolutely ruled out in view of the brittle fracture tendency of materials used to date for cylinders or pipes under pressure.

Tests have shown that a notch impact energy of 27 J at −20° C., as typically demanded heretofore, is not sufficient for standardized specimens to absolutely rule out structural failure as a result of brittle fracture at this temperature.

Comparative systematic tests of ready-for-use cylinder pipes, that include notch impact bending tests, drop weight tear tests, and structure tests have shown that a substantially ductile structural failure can be expected only when the notch impact energy is at a minimum value commensurate with a shear fracture area of 50% in the DWT test.

This means for example for a St52 that the values to be attained in the notch impact bending test need to have a minimum value of about 80 J at operating temperature to provide the structural part with enough plastic deformation reserves to prevent the risk of a brittle, multipart disintegration of the structural part.

The notch impact energy determined in the notch impact bending test for the finished pipe cannot be raised to the necessary level by the currently employed manufacturing process.

It is an object of the invention to provide a process for manufacturing cold-formed, in particular cold-drawn precision steel pipes for application in particular as pressure-operated cylinder pipes, to positively ensure a substantially ductile failure of the pipe in a simple and cost-efficient manner, even at operating temperatures of up to −20° C.

Based on the preamble, this object is attained in combination with the characterizing features of claim 1. Advantageous improvements are the subject matter of subclaims.

According to the teaching of the invention, a process is applied in which the pipe blank is finish-drawn in one or more passes, wherein the pipe undergoes a heat treatment before finish-drawing, and the steel pipe has the following chemical composition (in %):

C = 0.05-0.25 Si = 0.15-1.0  Mn = 1.0-3.5 Al = 0.020-0.060 V max. 0.20   N max 0.150 S  max 0.030, with optional addition of one or more alloying elements such as Cr, Mo, Ni, W, Ti, or Nb as well as impurities caused by melting.

The optional addition of the alloying elements is dependent on the required property profile, i.e. according to the desired mechanical properties, and have advantageously the following contents (in weight-%):

Cr max. 0.80 Mo max. 0.65 Ni max. 0.90 W max. 0.90 Ti max. 0.20 Nb max. 0.20.

The heat treatment itself includes a classical hardening with subsequent tempering of the pipes. Austenitizing is carried out at temperatures of about 910-940° C. depending on the respective material, followed by a quenching process to form a hardening structure. Quenching may be executed using various quenching media, typically quenching is implemented by means of water using a water shower. When using air-hardening materials, cooling may be realized through exposure to static air.

Tempering treatment follows hardening and is carried out at temperatures of about 540-720° C. depending on the material.

The advantage of the proposed process is the realization of a very even homogenous microstructure with superior toughness by providing a heat treating step before the finishing pass, which microstructure is substantially maintained even after the finishing pass of the pipe. Tests have shown that the values for the notch impact energy at −20° C. and 50% shear fracture area in the DWT test lie for transverse test specimen at a superior 80 J and for longitudinal test specimen at 100 J.

A possible demand by customers for a final annealing in the form of a stress-free annealing after the finishing pass leads to an additional improvement of the notch impact energy values and thus toughness of the structural part.

The final annealing is carried out advantageously at a temperature range of 600-700° C. in dependence on the material, whereby care should be taken that the temperature should be precisely set in dependence on the material properties to be attained, like e.g. strength, elongation at fracture, and notch impact energy.

Test of pipes made in accordance with the process according to the invention have shown the elimination of the otherwise typically encountered ferritic-pearlitic microstructure of the construction steels with pronounced variations in the notch impact energy level in transverse as well as longitudinal test specimens in materials produced by the process according to the invention.

This is clearly shown by the test results for notch impact energy values on cylinder pipes of StE 460 mod., as illustrated in FIG. 1. An almost identical notch impact energy level of up to about 180 J is reached in longitudinal as well as transverse direction.

As illustrated in FIG. 2, the structural parts made from the steel pipe StE 460 mod. in accordance with the invention have, compared to the steel pipe produced in a conventional manner, a sufficiently high proportion of ductile fracture behavior at temperatures of up to −20° C., and thus have sufficient plastic deformation reserves to positively prevent the risk of a disintegration of the structural part into several parts.

The material concept according to the invention thus allows the operation of hydraulic cylinders even at the temperature range of up to −20° C.

In certain steel grades, there is the positive side effect of a significant increase of the strength values. This allows advantageously a reduction in wall thickness of the cylinder pipes by about up to 30% and thus a reduction in weight, satisfying the demands of lightweight construction.

In summary, it should be noted that the process in accordance with the invention for manufacturing cylinder pipes subject to pressure positively prevents a multipart structural failure even at operating temperatures of up to −20° C. and moreover permits a reduction in wall thickness of the cylinder wall of up to 30%. 

1-10. (canceled)
 11. A process for manufacturing a precision steel pipe having the following chemical composition (in weight-%): C = 0.05-0.25 Si = 0.15-1.0  Mn = 1.0-3.5 Al = 0.020-0.060 V max. 0.20   N max 0.150 S max 0.030

and impurities caused by melting, said process comprising the steps of: drawing a pipe blank with defined starting condition in at least one pass; subjecting the pipe blank to a heat treatment; and subjecting the pipe blank to a finishing drawing step to produce a finished pipe.
 12. The process of claim 11, wherein the pipe blank is a seamless, hot-formed pipe blank.
 13. The process of claim 11, wherein the pipe blank is welded pipe blank made from a hot strip.
 14. The process of claim 11, further comprising the step of adding at least one alloying element selected from the group consisting of Cr, Mo, Ni, W, Ti, and Nb.
 15. The process of claim 11, further comprising the step of adding alloying elements in a following composition (in weight-%): Cr max. 0.80 Mo max. 0.65 Ni max. 0.90 W max. 0.90 Ti max. 0.20 Nb max. 0.20


16. The process of claim 11, wherein the heat treatment comprises the step of heating the pipe blank to a temperature of in a range of 910-940° C., cooling the pipe blank, and exposing the pipe blank to a tempering treatment.
 17. The process of claim 16, wherein the cooling step comprises accelerated cooling.
 18. The process of claim 17, wherein the accelerated cooling step comprises quenching.
 19. The process of claim 11, wherein the quenching step is implemented by means of a water shower.
 20. The process of claim 16, wherein the cooling step comprises exposure to static air.
 21. The process of claim 16, wherein the tempering treatment is carried out at a temperature range of 540 to 720° C.
 22. The process of claim 11, further comprising the step of subjecting the finished pipe to a final annealing.
 23. The process of claim 22, wherein the final annealing step is carried out at a temperature range of 500 to 700° C. 